The risk is that v0.7 nodes would be vulnerable to attacks by double spends, 51% attacks and accepting newly generated coins from the incompatible v0.7 generation blocks.
Actually this is incorrect. The Satoshi client goes into safe mode when it notices that a branch it believes is invalid has become the longest one by more than 6 blocks. This actually happened to the 0.7 clients, which is what Pieter Wuille is talking about here in the original fork announcement:
If you're on 0.7 or older, the client will likely tell you that you need to upgrade.
Some more details and links to the code are
here if you are interested.